Creating efficient and controllable, insect sized NAVs that are capable of controlled, hovered flight and terrestrial locomotion is proving a considerable challenge. A NAV's scale as generally understood in the industry is defined as being less than 7.5 cm wingspan and 10 g in weight, however current design efforts are aimed at smaller scales less than 3 cm wingspan and 1 g in weight.
A NAV engine's thrust-to-weight ratio needs to be high enough as is required to lift the power source and control electronics, this being increasingly difficult to achieve with decreasing scale due to the non-linear scalability of component power densities. To keep the thrust-to-weight ratio high and power consumption low, it becomes increasingly important to have efficient motor and transmission systems for both aerial and terrestrial locomotion. As the NAV scale decreases, the wing beat frequency often needs to be disproportionably increased to maintain comparable performance. Higher wing beat frequencies offer more efficient lift and improved flight stability in windy conditions but the increased frictional energy losses in the motor and transmission systems have a detrimental consequence for efficiency, and engine longevity as related to structural fatigue engine failure.
Typical NAV research and prototypes for micromechanical insect engines less than 100 mg employ the use of complex kinematic pair transmissions, with flexure lever joints to amplify the small deflections from one or more suitable electromechanical transducers, transforming electrical energy into complex wing kinematics suitable for insect inspired flight. Piezoelectric materials, shape memory materials, dielectric elastomer and electrochemical actuators are amongst the electromechanical transducers being explored.
The capacity of the NAV's battery or cell needs to be such that the NAV can perform useful flight durations. For a remote control indoor toy fly, 2-3 minutes of flight may suffice but for commercial aerial photography, flight duration times of one hour plus would be beneficial. Therefore, for most practical uses the power source is a heavy component to lift.
The difficulties in creating sufficient thrust-to-weight engine performance, at progressively smaller scales of typically less than 3 cm wingspan, is driving a considerable effort in the industry, to reduce motor, airframe and transmission weight whilst increasing strength and power density.
Typically, when a NAV's required wing kinematic uses more degrees of motion to implement flight control parameters, further transmissions, actuators and associated electronics are added, so adding to the engine's weight and reducing its power density. If more appendages, such as legs with terrestrial locomotion, are added, this again can substantially increase the weight the NAV must sustain in flight.
Operability under natural flight conditions of rain, dust, heat and cold is another area of concern. Flexure-lever kinematic pair transmissions can easily suffer particle damage from sand and dust unless in a protective enclosure that adds weight. Temperature fluctuations and rain effects on transmissions can also prevent flight. Currently, NAV scaled ornithopter mechanisms use some form of kinematic pair transmission with fairly constrained degrees of movement; thus, they tend to suffer from increased friction, due to higher bearings loads from constrained incompliant moments of torque. In addition, such mechanisms require a strong, and therefore relatively heavy, airframe on which to mount the kinematic pair transmission(s) for reaction there against. These flapping systems often benefit from operating at resonance, which reduces frictional losses and means that the systems operate with greater efficiency than otherwise. However, due to the constrained movements of the kinematic pair transmission(s), the systems do not directly benefit from resonance to amplify and convert efficiently small strains to large strains of desired wing kinematics. Rather, the amplification of movements, from small deflections of an actuator to larger deflections of a wing, is achieved mechanically through the flexure-lever kinematics.
Other known NAV devices use chemical actuation to operate the appendages (wings).
These engine design challenges noted above are preventing the emergence of a practical NAV.
Resonant frequencies are normally avoided in any physical product, as they can lead to catastrophic structural failures. However, by appropriately controlling mechanical resonance through frequency and amplitude modulation, useful mechanical deflections of specific magnitude and direction can be generated on demand for powering wings and/or legs on a micromechanical insect NAV.